The prior art method for detecting the absolute position of a revolving device, such as a motor, a mechanical rotary shaft, etc., includes the use of a rotary shaft which is provided with a coding device consisting of a set of sensing element for sensing a set of binary identification code corresponding to the position, as shown in FIG. 1. In order to bring about the binary identification code of such absolute position, the prior art method makes use of the optical absolute position coding device and the drum-type absolute position coding device. The binary identification code is brought about by the signals representative of the open (O) and the close (I) actions of the sensor element. The position of the revolving device is known after reading.
As shown in FIG. 2, the prior art optical absolute position coding device is composed of a signal disk 10, an optical hole disk 11, a light-emitting diode 12, a signal receiver 13, and a rotary shaft 14. The signal disk 10 is provided by etching with a plurality of penetration portions 101 and solid portions 102, which are arranged circularly and alternately, as shown in FIG. 3. When the signal disk 10 is shone by the light-emitting diode 12, a portion of light is allowed to pass whereas a portion of light is obstructed, in light of the effects of the penetration portions 101 and the solid portions 102. The light that is allowed to pass is received by the signal receiver 13 for bringing about the binary identification code. After detection, the position of the revolving device is determined.
Such a prior art optical coding device as described above has several shortcomings, which are described hereinafter.
The precision of the coding device is dependent on the precision of the signal disk 10 which must be thus made with precision at a high cost. In other words, the prior art optical coding device is not cost-effective.
In view of the fact that the coding devices of various resolutions are used in the sites of various situations, the signal disks 10 of various specifications with respect to precision must be made, thereby resulting in a substantial increase in the capital expenditure.
The operation of the optical coding device of the prior art is vulnerable to interference caused by the environmental factors, such as grease, humidity, dust, etc.
As shown in FIG. 4, the drum-type magnetic absolute position coding device of the prior art is composed of a magnetic drum 20, a rotary shaft 21, and a sensor 22. The magnetic drum 20 has an outer layer which is coated by ejection with a magnetic material layer 201 which is in turn magnetized with a plurality of magnetic pole loops 202, 203, 204 and 205, which are different in magnetic pole number from one another. Located among the magnetic pole loops 202, 203, 204 and 205 is a neutral area. Each of the magnetic pole loops 202, 203, 204 and 205 is corresponding to the sensor 23 of the sensor element set 22. The position of the revolving device is identified by the output signal of the sensor element set 22.
The above-described drum-type magnetic absolute position coding device of the prior art is defective in design in that the magnetic material layer 201, which is coated by ejection on the outer layer of the magnetic drum 20, must be finished at an extra cost. In addition, the magnetic pole loops and the neutral area must be finished with precision at a high cost.